struct TreeNode {
  int val;
  TreeNode *left;
  TreeNode *right;
  TreeNode()
      : val(0), left(nullptr), right(nullptr){} * TreeNode(int x)
      : val(x),
        left(nullptr),
        right(nullptr){} * TreeNode(int x, TreeNode *left, TreeNode *right)
      : val(x),
        left(left), right(right) {}
};

int countNodes(TreeNode *root) {
  if (!root)
    return 0;
  return countNodes(root->left) + countNodes(root->right) + 1;
}